Oil quality sensor measuring bead volume

ABSTRACT

Method and apparatus for determining deterioration of e.g. lubricating oil by measuring the contraction of a polymeric matrix (support) holding charged ionic groups. The support contracts due to oil quality degradation. The expansion/contraction is detected e.g. electrically.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/637,878 filed Apr. 25, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to measurement and testing for liquid analysis,and more particularly to determining electrically the quality of anynatural or synthetic oil, oil substitute, oil including additives, orany other non-polar or weakly polar liquid.

2. Description of the Related Art

Commonly invented U.S. Pat. No. 5,435,170, incorporated herein byreference in its entirety, describes a method and apparatus to determinethe quality of a fluid, e.g. oil, by its solvating effect on aninsoluble (resin) matrix to which charged ion groups have beencovalently bound. The solvating effect is measured as a variation in anelectrical characteristic (e.g. capacitance or conductivity orimpedance) of the matrix. The apparatus in one embodiment includes ahousing holding a conductive mesh containing small (milligram) amountsof ion-charged resin beads. A metal probe is fitted in the mesh andmakes contact with the resin. The entire apparatus is immersed in thefluid so the fluid enters the housing, and the electrical characteristicis measured from the probe to the mesh through the resin. Fluid qualitydegradation is measured as a change in electrical conductivity orcapacitance through the resin with respect to an increase in the fluid'ssolvent polarity.

The present inventors have discovered several improvements for theirearlier oil quality sensor. These improvements improve the utility andapplications of the oil quality sensor which measures typically oiloxidation or additive depletion in real time and can be used to predictthe level of degradation of an oil over the normal operating temperaturerange of a vehicle. It has been found that it would be desirable toimprove and extend the utility of that oil quality sensor in terms ofthe chemistry of the resin beads, sensitivity and dynamic range by theof the sensor, oil contaminant detection, and to adapt the oil qualitysensor to become an oil level sensor.

SUMMARY

In accordance with the present invention, the expansion/contraction ofthe resin beads due to contact with oil of varying quality is exploitedmechanically. The beads are enclosed in a flexible enclosure; as theyexpand (or contract) the resulting change in size of the enclosure isdetected e.g. electrically, and used to indicate oil quality. In oneembodiment the enclosure is inside a spring loaded housing; as theenclosure contracts, the resulting change in length of the housing isdetected electrically by a sliding contact on a resistive memberattached to the housing.

Also in accordance with the present invention the method and apparatusdisclosed in the above-referenced patent, which use a matrix ofpolystyrene (resin) beads to conduct a charge when the amount ofconductivity and/or capacitance reflects the condition of the oil, firstis subject to a chemical process improvement whereby a polar environmentis created locally around the charged groups of the resin beads. Thispolar environment promotes charge separation or dissociation between thecation and anion of the charged groups. This amplifies the signal andimproves the sensitivity of the sensor. This is accomplished bypretreating the resin beads with a high boiling point polar proticsolvent, such as ethylene glycol.

Also in accordance with the present invention, the dynamic range of thesensor is improved by exploiting the fact that the physical size of thebeads changes (becomes smaller) as the fluid changes from a non-polartowards a polar type fluid. The decreasing bead size has been found todecrease the physical contact between the charged groups on the beadswhich in turn causes a change in the electrical properties (e.g. adecrease in conductivity).

Also in accordance with the invention, the beads are housed in apermeable container so as the beads decrease in size (due to the fluidchanging to be more polar) they will filter through the container, sosensor sensitivity is improved. In one embodiment multiple sensorchambers are used with intervening filters each having specific meshsizes to measure specific levels of fluid polarity.

It has also been found that the beads may be loaded into the sensor atan initial particular size, such that after being immersed in a morenon-polar liquid (e.g. oil) the beads expand, forcing better physicalcontact between them.

In another embodiment, the beads are loaded into the sensor in a highlyswollen state and as the fluid (e.g. oil) in which they are immersedbecomes more polar their conductivity decreases. Other modifications arealso possible utilizing bead swelling and shrinking depending on thecondition of the surrounding fluid (e.g. oil). For instance, a springloaded diaphragm may push on the beads thus maintaining their electricalconductivity. The travel limit of the spring can be set such that itwill stop before the beads are at their minimum diameter (correspondingto a specific polar condition). This configuration allows the signal toremain relatively constant over a predetermined range of polarity (cleanoil to a specified wear state). Once the bead diameter contracts(corresponding to a further change in polarity) such that the springtension is no longer requiring physical contact of the beads with eachother, the electrical conductivity of the system will degrade rapidly.

In accordance with another aspect of the present invention, it has beenfound when the oil contains heterogenous contaminants in the form ofbubbles or droplets that have a different polarity than does the oil,i.e. when water, anti-freeze, fuel, or even metals become mixed with theoil, the transient behavior of the droplets as they pass through thesensor generates electrical noise in the sensor output signal. Thefrequency and amplitude of the noise is directly correlated to theamount of contamination, and hence heterogeneous oil contamination bywater or other substances is detectable.

Also in accordance with the invention, if the contaminating materialhappens to be mixed uniformly into the oil to form a homogenous fluid,this is sensed by providing two sensors with beads each having beenpretreated with different materials (e.g. polar protic solvents such asethylene glycol and methanol). Over the course of oil degradation, oneset of treatedbeads/sensor causes a decrease in conductivity while thesecond set of treated-beads/sensor causes an increase in conductivity.Thus when the conductivity measured by the two sensors diverges thisclearly indicates contamination.

Also in accordance with the present invention, the beads in two sensorsare treated with two different cations or other charged groups, whichgives the electrical output signal of each sensor a specific signatureas it tracks oil degradation, and the output signals of the two sensorsare compared. For instance, iron or lead is thereby easily sensed in theoil indicating excessive engine wear and/or oil degradation.

In accordance with yet another aspect of the present invention, amulti-chamber sensor with the chambers arranged vertically, eachcontaining charged groups in a matrix and its own sensor electrode(s),is used to sense a level of the oil, for instance in the oil pan of anautomobile, to determine a low oil condition. The chambers arepositioned so that they are at various levels in the oil pan; theabsence of oil from any one of the chambers is easily sensedelectrically due to the conductivity difference compared to a referencesensor constantly immersed in the oil.

It is to be understood that while the following description is of an oilsensor, the invention is not so limited, and includes method andapparatus for sensing properties of other suitable fluids and also isnot limited to automotive or motor applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an oil quality sensor assembly fromthe above-referenced patent and also in accordance with this invention.

FIG. 2 is a schematic diagram of the oil quality sensor assembly andassociated circuitry from the above-referenced patent.

FIG. 3A shows a three-chamber oil sensor in accordance with the presentinvention.

FIG. 3B shows the chamber of FIG. 3A with the beads in the centralchamber.

FIG. 3C shows the chamber of FIG. 3A with the beads in the lowestchamber.

FIG. 4 shows an oil quality sensor device having two sensors withdifferent properties.

FIG. 5 shows an oil level sensor with multiple chambers in accordancewith the present invention.

FIGS. 6 and 7 show respectively front and back views of an oil qualitysensor that measures bead volume change in accordance with the presentinvention.

DETAILED DESCRIPTION

An oil quality sensor in accordance with the present invention is inmany respects similar to that disclosed in the above-referenced U.S.Pat. No. 5,435,170. Hence present FIG. 1 is a cross-sectional view of anoil quality sensor identical to FIG. 1 of that patent in which oilquality sensor 1 is mounted in an otherwise conventional drain plug 2used in the oil pan of an internal combustion engine i.e. an automobileengine. The drain plug 2 with its standard hex nut arrangement andassociated threaded surface 3 are shown as the chief mounting for theoil quality sensor. The e.g. plastic housing 4 provides the outercontaminant for conventional stainless steel wire mesh 6, which in turnholds the polystyrene resin beads 8 impregnated with the charged iongroups, in one embodiment sodium as the cation and sulfite as the anion,with the sulfite covalently bound to the beads. A typical amount ofbeads is 20 to 500 mg (not limiting). The beads are e.g. cross-linkedwith 8% cross-linked divinyl benzene and have a titer or exchangecapacity of 1.7 meq/m1, each bead being of e.g. 16-400 wet mesh size(1,180 to 38 μm diameter). (The divinyl benzene may be e.g. 0% to 12%cross-linked). Further details of the beads are disclosed in theabove-referenced patent application. Also, other types of support andother suitable cation exchange groups may be used.

Opening 5 allows oil to flow through the mesh 6 and the resin beads 8.Although not shown, a similar opening on the opposite side of a housing4 allows a flow-through arrangement. The metal probe 7 is one electrodeof the electrical circuit for measuring the desired electricalcharacteristic through the resin matrix. Wire 13 is connected at point12 and routed to the external plug 11 via a conventional oil-tight seal(not shown). Wire 10 is connected to the mesh 6 at point 9 and routed tothe external plug 11, also via an oil-tight seal (not shown). It can beseen that the mesh is the second electrode of the electrical circuit formeasuring the electrical characteristic through the resin matrix. Plug11 connects the sensor to the external signal conditioning circuit.

One variant of the sensor of FIG. 1 (not shown) includes a smallnonconductive strip of material (e.g. plastic) defining two holes. Thestrip is covered on each side with a mesh of e.g. stainless steel cloth;each hole holds a quantity of the beads. A cover defining several slotsencloses this assembly, which is mounted in an oil pan drain plug. Whileone hole in the strip allows the engine oil to flow freely through thebeads, the other hole is permanently sealed with clean oil trappedinside to act as the internal standard (reference). Thus electricalconnections are provided to the beads in each hole.

The slots in the cover in one embodiment are offset (located away from)the bead location to minimize oil flow past the beads, thereby toprevent loss of bead physical contact due to oil circulation.

FIG. 2 is a schematic diagram of the oil quality sensor system of theabove-referenced patent, herein measuring conductivity as anillustration. Other electrical characteristics could be usedalternatively, such as capacitance. The positive voltage V+ at node 15causes an electric current to flow through a voltage divider consistingof resistor 14 and sensor element 1. Coaxial cable 16 reduces the effectof outside electrical noise on the signal from the remotely locatedsensor element 1. The resulting voltage developed across the sensorelement where the other terminal of coaxial cable 16 is connected toground at node 17, which is referenced to voltage V+ is applied to thenoninverting input of the voltage follower circuit 18.

Voltage follower circuit 18 is a high input impedance amplifier, such asan RCA CA3140 integrated circuit. Such a voltage follower circuit isused because of the very high impedance exhibited by the sensor elementunder normal operating conditions; any loading of the circuit by anexternal measuring means would affect the accuracy of the subsequentvoltage readings.

The voltage output at node 19 from voltage follower 18 as shown isapplied to a conventional signal interface circuit 20. Details of thesignal interface are given in the above-referenced patent and mayinclude for instance a simple analog meter, or an analog to digitalconverter whose output signal is supplied to a microprocessor system forfurther conditioning and subsequent output to a conventional display.

In a first improvement in accordance with the present invention, a polarenvironment is created locally surrounding the charged groups of thebeads. ("Beads here refers generally to the support, of whateverstructure.) This polar environment promotes charge separation ordissociation between the cation and anion of the charged groups. Thisamplifies the signal and improves the sensitivity or resolvingcapabilities of the sensor. Creating and maintaining a polar environmentaround the charged groups in a non-polar environment (e.g.uncontaminated clean oil) is accomplished in accordance with the presentinvention by pretreating the beads (before being loaded into the sensor,i.e. during manufacture of the sensor) with e.g. a high boiling pointpolar protic solvent such as ethylene glycol. A protic solvent formshydrogen bonds with the sulfonyl group of sulfonic acid salt (cation)even at temperatures of up to 1500° C., thereby creating the desiredlocal polar environment. This has been found to increase resolution bycreating an amplified sensor output signal.

The actual process of preparing the beads involves washing the beadswith 1N sodium hydroxide for about 15 to 30 minutes at room temperature.The excess sodium hydroxide is washed off in methanol. The beads arethen soaked in methanol to remove any excess water, then air dried toremove any remaining methanol. The beads are then soaked in the ethyleneglycol for e.g. 24 hours, and heated to 120° C. for about 2 hours toensure penetration of the ethylene glycol. The beads so treated arefully swollen.

Last, the beads are placed in clean oil (or another non-polar fluid) andheated to 120° C. to remove any excess ethylene glycol and shrink thebeads down to their "clean oil" state. The beads are then loaded intothe sensor under slight pressure so they are in close contact with oneanother. Alternatively, the beads are soaked in the ethylene glycolafter being loaded into the sensor.

Beads so treated have been found advantageously to improve the signalstrength from the sensor one to two orders of magnitude. What isoccurring in terms of electrochemistry is that the hydrogen of theethylene glycol bonds to the oxygen of the bead resin, creating thedesired localized polar environment. The relatively high boiling pointis desirable because lower boiling point substances might boil off whenthe oil becomes hot in the engine environment.

In accordance with another aspect of the present invention, the presentinventors have used the extent of bead swelling and shrinkage todetermine solvent/liquid polarity. (Cross-linking of the beads increasesrigidity of the resin matrix and increases the number of divinyl benzenegroups.) In this approach the beads shrink in size with increasing oilpolarity (i.e. degrading of the oil) and the beads then physically passthrough a mesh (filter) and collect in a lower chamber. The collectingchamber produces an electrical signal from its electrodes as affected bythe quantity of beads present, which serves to confirm the measuredoxidation of the oil. Hence it has been found that as the fluid changesfrom a non-polar towards a polar one, the diameter of the beads exposedto the oil becomes smaller (the beads shrink). Decreasing the size ofthe beads has been found to affect the contact between the chargedgroups which in turn changes the electrical properties of the beads(e.g. decreases conductivity). By selecting for instance a permeablecontainer containing a fine meshed filter which corresponds to the sizeof the beads, one may insure that when the beads are in a non-polarenvironment (clean oil) they will not pass through the filter.

An example of such a sensor is shown diagrammatically in FIG. 3A where athree-chamber housing 30 for the oil quality sensor has chambers 32, 34and 36. Initially the beads 40 (with the charged groups) are located inchamber 32 and are held therein by a suitable filter or mesh 44. As theoil 48 (which circulates through all three chambers 32, 34, 36)degrades, i.e. becomes more polar, the bead diameter decreases and dueto gravity the beads 40 pass through the filter 44 into the secondchamber 34, as shown in FIG. 3B, thereby causing a decrease inconductivity in the first chamber 32 as the beads 40 are eliminatedtherefrom. There is a corresponding increase in conductivity in thesecond chamber 34 as the beads 40 enter that chamber 34. It is to beunderstood that each chamber 32, 34, 36 is equipped with two electrodes(not shown), for instance a probe and a second electrode which might bethe filter mesh. The corresponding signal processing circuitry is notshown but its nature will be readily understood by one skilled in theart in light of the above-referenced patent.

As the beads 40 pass, under the influence of gravity and the oilcirculation, from the first chamber 32 down to the lower second chamber34, there is a substantial decrease in the strength of the conductivitysignal across the electrodes of the first chamber 32 versus those of thesecond chamber 34. This greatly increases the sensing dynamics of thesystem and makes signal sensing relatively easy. This method also allowsdetection of oil reaggregation, (the onset of oil sludge). This isbecause the present inventors have found that as the oil reaggregates,the oil's solvent properties change from a polar state back to anon-polar state which is indicated by an increase in conductivity in anyone chamber.

Multiple chambers can be used with a third chamber 36 separated from thesecond chamber by another filter 50 having yet a finer mesh capacity tomeasure further shrinkage in the beads 40. As the oil further degrades,the beads 40 pass from the second chamber 34 into the third chamber 36as shown in FIG. 3C, and the signal strength across the electrodes inthe third chamber 36 is measured to indicate this state of the oil. Ineffect, this multi-chamber oil sensor is an "electromechanical statemachine" where the various oil states are determined, not only by thecurrent polar condition of the oil, but also by the previous polarcondition of the oil. The state of the oil hence can be measured bymonitoring the electrical characteristics of each chamber via itsassociated electrodes, either directly or through an interveningprocessor such as a microprocessor programmed to interpret the relativesignal strength from the electrodes associated with each chamber anddrive a display (e.g. an LED or LCD) indicating visually thecorresponding oil condition.

With the multiple chamber approach, the initial size of the beads iscontrolled to be within a small range to correspond to the initial meshsize. Also for use in a diesel engine (where often soot is present inthe lubricating oil) one uses a larger bead size and a larger mesh. Thisprevents the soot from clogging the mesh.

Also in accordance with the invention, the beads are loaded into thesensor (in this case a single chamber sensor) initially in a non-swollenstate such that after being immersed in a more non-polar liquid (cleaneroil, or an oil that contains a higher quantity of non-polar additives),the beads expand, forcing physical contact amongst themselves. Theresulting electrical signal from the sensor electrodes remains at thesame level of conductivity until the bead's initial loading polar statewas achieved again, to indicate a certain degree of oil degradation.Only then would the conductivity be allowed to decrease by separation ofthe beads due to less physical contact in a more polar condition of theoil. This provides a definite indication of a particular degree of oildegradation.

Conversely, in another version the beads are initially loaded into thesensor in a highly swollen state. After the oil degrades and hencebecomes more polar, the sensor conductivity shows a more immediatedecrease, hence providing a sharply defined indication of oildegradation and an easily interpreted output signal.

In a third approach utilizing bead swelling and shrinking, the beads areloaded into the sensor to a specified loading quantity (not completelyfilling the chamber) in the least swollen state. This least swollenstate is obtained by initially immersing the beads in a highly polarsolution before loading them into the sensor. When the sensor in use isexposed to oil that is less polar, the beads expand, increasing sensorconductivity. Since the additives used in commercial oil change theoil's polarity, the sensor can be loaded initially with beads swollen toa size such that in oil without additives, the sensor is at a known filllevel of beads. The sensor can then be placed in the oil with additivesin the engine. The change in conductivity due to the beads swelling andshrinking then tracks additive addition and depletion, e.g. in alaboratory environment.

In yet another aspect in accordance with the present invention, thepresent inventors have found when the oil contains localizedcontaminants which form droplets having a different polarity than doesthe oil (contaminants such as water, anti-freeze, fuel, or even metals)the transient behavior of the heterogenous contaminant droplets as theypass through the sensor causes easily observable electrical noise in theoutput signal from the sensor electrodes. This would typically occurwith a major coolant leak into the engine. The frequency and amplitudeof the noise is directly correlated to the quantity of contamination.Quantities of such heterogenous contaminants as little as 2% can beeasily detected using this approach. In this case the oil sensor is asshown in FIGS. 1 and 2 with additional output signal processing. Thissignal processing circuitry looks for noise of a particular amplitude orfrequency in the output signal to detect contamination. One signalprocessing approach is to observe a standard deviation of the signalamplitude or of the signal frequency; this is easily performed bycircuitry or in software using a microprocessor connected to the sensoroutput circuitry, to provide a warning of oil contamination upondetection of a particular frequency or level of electrical noise.

Some contaminating materials may be emulsified into the oil and hencenot detectable by this approach. This could occur under some conditionsdue to water condensation, anti-freeze or fuel contamination. In thiscase, another approach involves chemically treating the resin beads intwo sensors with different charged groups, such that with one chargedgroup conductivity is increased by the contamination while the secondcharged group in the second sensor beads causes the conductivity todecrease. Both sensors are placed in the oil and their output signalscompared. Therefore when the conductivity of the two sensors diverges,contamination is indicated, where the amount of the divergencecorrelates to the amount of contamination.

This sensor arrangement is shown in FIG. 4 with two sensors 60, 68. Thebeads 64 in sensor 60 are treated with e.g. sodium as the cation. Thebeads 70 in sensor 68 are treated with e.g. iron as the cation. Bothsensors 60, 68 are exposed to the oil, and the output signals from theirelectrodes (not shown) are both routed to signal processing circuitry74, for comparison by comparator 76, the output of which is provided tooutput circuitry 80 for display to indicate oil contamination. It is tobe understood that FIG. 4 is explanatory and not limiting of the twosensor approach.

In accordance with another aspect of the present invention, before beingplaced into the sensor (during manufacture) the beads are initiallytreated with a particular charged group, e.g. sodium, which gives theelectrical signal a specific signature as it tracks oil degradation.When the sensor is in use, the sodium is displaced by other metal ionsfrom the oil that change the signature of the output signal. Ofparticular interest would be the electrical signature of iron or leadpresent in the oil, indicating engine wear due to iron or lead enteringthe oil from the engine components. Using two differently treatedsensors both immersed in the engine oil, such as one treated with sodiumand the other treated with iron, the output signals (electricalsignatures) of the two sensors are compared for instance by amicroprocessor. A sensor of this type would appear to be similar to thatof FIG. 4. Initially the signatures of the two sensors would bedifferent. When iron contamination of the oil occurs, the sodium ion inthe resin beads in the first sensor is displaced by the iron ion and thesignatures from the two sensors would then be the same, indicating oildegradation. The engine iron, copper, lead, zinc, etc. present in theoil is oxidized from a metal state to an ionic state by a compatiblecharged group and encounters the sodium on the resin beads and displacesit. A zero output from the comparator (see FIG. 4) indicatescontamination for instance by copper, zinc, lead, or iron, and excessiveengine wear.

One could even determine where the particular wear is occurring in termsof engine components, depending on e.g. the known lead concentration ineach various engine component.

Also in accordance with the present invention a multi-chamber sensordetermines oil level, typically to determine a low oil condition. Inthis case the entire sensor assembly 90 as shown in FIG. 5 is located inthe engine oil pan 98 and arranged vertically. The indicated oil levelsare e.g. the oil pan 98 is full, one quart low, or the danger level atwhich the engine is subject to damage. In this case each sensor chamberS1, S2, S3 is physically isolated from the other and each has its ownbeads 92, 94 and own sensor electrodes (not shown). Sensor S3 is alwaysimmersed in oil and serves as a reference. Hence each chamber by itselfis similar to the structure of FIG. 1. It has been found that the signalfrom a particular sensor is much different when it is immersed in oilcompared to when it is in air (when there is no oil at that level). Thusby positioning the chambers S1, S2 at various levels in the oil pan, theoil level can be determined. The sensor electrodes of chambers S1, S2,S3 can be connected electrically in series or in parallel and electricalcharacteristics of the chambers S1, S2, S3 are measured eithercollectively or individually.

Thus, in one example a direct current resistance measurement is taken.The typical resistance of clean oil is about 2M ohms at engine operatingtemperatures. The typical resistance of the beads 92, 94 in air is about10M ohms. If the electrodes of the two sensors S1, S2 are connectedelectrically in series the total resistance R is simply that of tworesistors in series: R=R₁ +R₂

For a full oil pan hence the resistance R is: R=2M ohms+2M ohms=4M ohms.

For an oil level that is one quart low, the resistance R is: R=2Mohms+10M ohms=12M ohms.

For the oil level that is dangerously low, the total resistance R is:

R=10M ohms+10M ohms=20M ohms.

In this case, one has three distinct different resistances depending onthe oil level to be compared to the resistance of the reference sensorS3. The various chambers need not be connected mechanically (need notshare the same housing) although, typically for simplicity they wouldshare a single housing as illustrated in FIG. 4. Also, the sensorsalternatively are connected electrically in parallel.

As described above, the resin beads are composed of two functionalgroups; 1) a hydrophobic component (i.e., the polystyrene/divinylbenzene hydrocarbon backbone) and 2) a hydrophilic component (i.e., theionic sodium/sulfonyl bond). The two groups differ in terms of theirsolvent properties. The hydrophobic component is attracted to orsolvated by a nonpolar environment while the hydrophilic component issolvated by a polar environment. Solvation typically results in a samplefreely dissolving into solution. In the case of the resin beads, theinsoluble nature of the resin matrix prevents the groups attached to thebeads from dissolving. Instead, the attached groups expand into thefavorable environment (as if dissolving) and take on the behavior ofswelling.

Either the hydrophobic or hydrophilic component can contribute to beadswelling. In the case of oil degradation, a clean or nonpolar oil causesbead swelling by solvating the hydrophobic components of the resin. Asthe oil degrades, the solvent properties change from a nonpolar natureto a polar one, which translates to a loss in salvation and results inbead shrinkage.

Pretreating the beads with ethylene glycol as described above solvatesthe hydrophilic group. Solvating the hydrophilic portion of the resinhas been found to enhance the hydrophobic swelling properties of thebeads. Solvating both groups results in effectively extending theswelling ranges of the beads. Ethylene glycol represents a significantimprovement in the operation of the sensor by improving both theelectrical characteristics and the mechanical (swelling) aspects of theresin used in detecting a change in the solvent properties of an oil.

In accordance with another aspect of this invention, the volume changeof the beads is measured, as opposed to the diameter change. Atemperature independent method of measuring volume change is used thatdoes not involve the electrical conductivity of the beads. Since thechange in volume is approximately four times the change in diameter(i.e., proportional to the cube of the radius), a change in volume iseasier to measure. Using this technique, the measurable range isincreased and the temperature dependence removed.

The change in volume is measured in one of a variety of ways in whichthe effect of temperature is negligible. One way is to pack a flexiblechamber which is permeable with the resin beads so that the chamber isexposed to oil and use a strain gage to measure the decrease inpressure. As the oil is oxidized, the beads shrink, causing a decreasein pressure; the pressure decrease directly relates to increasing oiloxidation.

A second method uses a sensor having a linear potentiometer attached toa spring tensioned plunger that is constantly compressing the beads. Thebeads are contained in a permeable chamber that is exposed to theoxidizing oil. As the plunger moves in response to the volume decrease,the change in resistance of the potentiometer is measured. As shown infront and back views in respectively FIGS. 6 and 7, the potentiometerresistive element 74 is a 1/16" diameter polysulfone rod 56 that hasbeen coated with a carbon (electrically conductive) paint. The housing60 is of a non-conductive plastic and the permeable chamber 63 is astainless steel mesh 82. Electrical measurement from one end of the rod56 to a sliding contact 62 on the housing 60 of the sensor shows thechange in resistance as related to the change in volume of the beads 66.

Compression spring 68 pushes apart the two ends of the housing 60. Formechanical purposes, rod 56 includes non-conductive member 56a, 56b ateach end. The resin beads are loaded into rod 56 at hole 70. Resistiveelement 74 lies between electrical leads 76, 78. Hence the beadexpansion/contraction is used in a purely mechanical sense to change thelength of the resistive element that is in the circuit between leads 76,78; the amount of resistance thereby indicates the fluid status. Thefluid (e.g., oil) enters rod 56 through mesh 82 defining flow slots 86so as to contact the resin beads.

In another embodiment, rather than electrically measuring the beadcontraction, the change in length of rod 56 is measured e.g. optically,using a linear scale and optical encoder.

In another version (not shown but otherwise similar to that of FIGS. 6and 7), a potentiometer is not necessary. Instead the plunger 74 is madeof a conductive material and the housing of a non-conductive material.At a specific point on the housing, a conductive pickup is mounted suchthat when the beads have decreased in diameter to a specific volume(relating to a specific oxidation state of the oil), the plunger thencontacts the conductive pickup, causing a signal to be transmittedindicating that the oil has degraded. As explained herein, many factorscontribute to electrical characteristics of the signals output by theoil quality sensor. The bead physical dimensions, the bead size, crosslinking of the beads, the initial loading concentration, and thepretreatment of the beads all contribute to the signal output. Inaccordance with the invention, these parameters are used in various waysin one or more oil quality sensor chambers to optimize the sensitivityof the sensor to various oil conditions for particular applications.Each sensor can be adapted as described above such to allow levelsensing in addition to e.g. metal detection and additive depletion.Thus, one may use various combinations of detection features formultiple detectors in a single sensor device.

This disclosure is illustrative and not limiting; further modificationswill be apparent to one skilled in the art in light of this disclosure,and are intended to fall within the scope of the appended claims.

We claim:
 1. A method of measuring an electrical characteristic of afluid, comprising the steps of:providing a quantity of resin beadsholding charged groups, the beads each being in a known state; exposingthe beads to a fluid; allowing a size of the beads to change uponexposure to the fluid, a change in the size of the beads moving a springloaded member; and measuring a position of the member.
 2. The method ofclaim 1, wherein the size of the beads increases.
 3. The method of claim1, wherein the size of the beads decreases.
 4. The method of claim 1,wherein the fluid is a non-polar or weakly polar fluid, and theelectrical characteristic indicates an increasing polar nature of thefluid.
 5. The method of claim 1, wherein the step of providing includesproviding the beads in a known swollen state.
 6. The method of claim 1,wherein the step of providing includes providing the beads in anon-swollen state.
 7. A fluid sensor comprising:an enclosure having atleast one opening for admitting a fluid into its interior; a quantity ofresin beads inside the enclosure, the enclosure having a spring loadedmember in contact with the beads, whereby exposure to the fluid changesa size of the beads, moving the member; and a mechanism coupled to themember and which measures a position of the member.
 8. The sensor ofclaim 7, wherein the mechanism includes an electrically resistiveelement contacted by a sliding contact coupled to the spring loadedmember.